1. Field of the Invention
The present invention generally relates to electro-optical readers for reading indicia such as bar code symbols and, more particularly, to an intelligent gain control system for such readers.
2. Description of the Related Art
Electro-optical readers typically employ an automatic gain control (AGC) circuit to maintain a desired constant signal envelope on an analog electrical signal generated by a sensor operative for detecting light scattered off indicia, e.g., a bar code symbol having a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics and being arranged in groups according to a set of rules and definitions specified by a code or symbology to form characters to be read.
This analog signal, also known as an analog bar pattern (ABP) signal, has voltage peaks corresponding to the edges between the bars and spaces of the physical symbol. The known AGC circuit includes a voltage peak detector for determining the peak-to-peak voltage of the ABP signal, and then alters the gain for the analog signal until the peak-to-peak voltage fits into a predetermined voltage range. Hence, AGC gain settings are largely determined by the maximum voltage peaks in the ABP signal and works well when the voltage peaks corresponding to symbol edges are relatively high compared to voltage peaks corresponding to non-symbol edges, for example, electrical noise peaks or specular reflection peaks, but works poorly when the non-symbol voltage peaks are significantly higher than the symbol voltage peaks. Indeed, the AGC circuit would set the gain too low to decode a symbol when the symbol voltage peaks are significantly smaller than the non-symbol voltage peaks.
FIG. 1 depicts a voltage-versus-time graph of an ABP signal generated by scanning a high contrast, low density bar code symbol in accordance with the prior art against a high contrast, uniform, non-specularly reflective background. The term “high contrast” refers to 80% minimum mean reflective distance (MRD), and the term “low density” identifies a symbol where the working range of the reader is limited by the amount of reflected signal and by the resulting signal-to-noise ratio, rather than being limited by the divergent focusing profile of the laser beam. Typically, a 55 mil dimension for a bar or space element is considered a low density symbol. The AGC circuit sets the gain for the ABP signal based on the detected maximum peak-to-peak voltage and works very well in decoding the symbol over working ranges over ten feet from the reader.
However, this nominal performance is degraded in the presence of specular reflections where there are a few very bright spots in the field of view of the sensor, the bright spots being brighter than the scattered light returned from the symbol. FIG. 2 depicts a voltage-versus-time graph of an ABP signal generated by the same symbol as discussed above for FIG. 1, but this time the ABP signal has specular reflections whose non-symbol voltage peaks are larger than the symbol voltage peaks. These larger non-symbol voltage peaks causes the AGC circuit to attenuate the ABP signal to the point where the symbol voltage peaks are too small to decode.
Another related example regards scanning a low contrast symbol, even if no specular reflections are present. Symbols having a low contrast between their bars and spaces have smaller voltage peaks in the ABP signal, requiring more gain, similar to a low density, high contrast symbol at the end of its working range. High contrast edges in the background of the symbol, but still within the scan line, can set the gain of the AGC circuit too low, again causing the AGC circuit to attenuate the ABP signal to the point where the symbol edge peaks are too small to decode.